High temperature flexible cementing compositions and methods for using the same

ABSTRACT

Natural fiber-containing cement compositions for cementing wellbores in high stress and high temperature environments. The cement compositions may contain natural mineral fiber materials such as wollastonite in an amount of greater than about 10% and in an amount selected to be effective to achieve ratios of flexural strength to compressive strength of cured cement that are greater than about 0.35 at downhole temperatures of greater than about 180° F.

[0001] This application claims priority on co-pending U.S. provisionalpatent application serial No. 60/269,153 filed on Feb. 15, 2001, thedisclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates generally to methods and compositions forcementing, and more specifically to methods and flexible cementcompositions for cementing in high stress and high temperatureenvironments.

[0004] 2. Description of Related Art

[0005] Cementing is a common technique employed during many phases ofwellbore operations. For example, cement may be employed to cement orsecure various casing strings and/or liners in a well. In other cases,cementing may be used in remedial operations to repair casing and/or toachieve formation isolation. In still other cases, cementing may beemployed during well abandonment. Cement operations performed inwellbores under high stress conditions may present particular problems,among other things, difficulty in obtaining good wellbore isolationand/or maintaining mechanical integrity of the wellbore. These problemsmay be exacerbated in those cases where wellbore and/or formationconditions promote fluid intrusion into the wellbore, includingintrusion of water, gas, or other fluids.

[0006] In a wellbore, cement may be used to serve several purposes.Among these purposes are to selectively isolate particular areas of awellbore from other areas of the wellbore. For example, in primarycementing, cement is commonly placed in the annulus created between theoutside surface of a pipe string and the inside formation surface orwall of a wellbore in order to form a sheath to seal off fluid and/orsolid production from formations penetrated by the wellbore. Thisisolation allows a wellbore to be selectively completed to allowproduction from, or injection into, one or more productive formationspenetrated by the wellbore. In other cases cement may be used forpurposes including, but not limited to, sealing off perforations,repairing casing leak/s (including leaks from damaged areas of thecasing), plugging back or sealing off the lower section of a wellbore,sealing the interior of a wellbore during abandonment operations, etc.

[0007] One important objective of a primary cement job is to providegood isolation between producing zones up to the surface and in a mannerthat will endure through the entire life of the well. No fluid movement,either gas or liquid, is normally desirable at any time through thecemented annulus. In this regard, possible paths for fluid movement inthe annulus include the interface between cement/rock and cement/casingand the cement matrix. Cement adherence to the formation and casing isprimary affected by cement shrinkage and by stress changes induced bydown-hole variations on pressure and temperature, especially inside thecasing but also at the formation.

[0008] Conventional well cement compositions are typically brittle whencured. These conventional cement compositions often fail due tostresses, such as compressional, tensile and/or shear stresses, that areexerted on the set cement. Wellbore cements may be subjected to shearand compressional stresses that result from a variety of causes. Forexample, stress conditions may be induced by relatively hightemperatures and/or relatively high fluid pressures encountered insidecemented wellbore pipe strings during operations such as perforating,stimulation, injection, testing, production, etc. Stress conditions mayalso be induced or aggravated by fluctuations or cycling in temperatureor fluid pressures during similar operations. Variations in temperatureand internal pressure of the wellbore pipe string may result in radialand longitudinal pipe expansion and/or contraction which tends to placestress on, among other things, the annular cement sheath existingbetween the outside surface of a pipe string and the inside formationsurface or wall of a wellbore. Such stresses may also be induced incement present in other areas of the wellbore in the pipe.

[0009] In other cases, cements placed in wellbores may be subjected tomechanical stress induced by vibrations and impacts resulting fromoperations, for example, in which wireline and pipe conveyed assemblyare moved within the wellbore. Hydraulic, thermal and mechanicalstresses may also be induced from forces and changes in forces existingoutside the cement sheath surrounding a pipe string. For example,overburden and formation pressures, formation temperatures, formationshifting, etc. may cause stress on cement within a wellbore.

[0010] Conventional wellbore cements typically react to excessive stressby failing. As used herein, “cement failure” means cracking, shattering,debonding from attached surfaces (such as exterior surfaces of a pipestring and/or the wellbore face), or otherwise losing its originalproperties of strength and/or cohesion. Stress-induced cement failuretypically results in loss of formation isolation and/or wellboremechanical integrity. This in turn may result in loss of production,loss of the wellbore, pollution, and/or hazardous conditions.

[0011] Injection or production of high temperature fluids may causethermal expansion of trapped fluids located, for example, between a pipestring and a cement sheath, between a cement sheath and the formation,and/or within the cement sheath. Such trapped fluids may createexcessive pressure differentials when heated and/or cooled, resulting incement failure. Thermal cycling (such as created by intermittentinjection or production of fluids that are very warm or cool relative tothe formation temperature), typically increase the likelihood of cementfailure.

[0012] In still other cases, mechanical and/or hydraulic forces exertedon the exterior of a cement sheath may cause stress-induced cementfailure. Such forces include, but are not limited to, overburdenpressures, formation shifting, and/or exposure to overpressured fluidswithin a formation. Increased pressure differential, such as may becaused when the interior of a cemented pipe string is partially orcompletely evacuated of liquid, also tends to promote cement failure,especially when combined with relatively high pressures exerted on theexterior of a cement sheath surrounding the cemented pipe string.

[0013] In addition, any type of thermal, mechanical or hydraulic stressthat acts directly on a set cement composition, or which tends to causedeformation of a wellbore tubular in contact with a set cementcomposition may promote, or result in, failure of a conventional cementcomposition.

SUMMARY OF THE INVENTION

[0014] Natural fiber-containing cementing systems and methods areprovided in which cement slurries may be formulated to provide hardenedcement compositions possessing relatively high resilience, elasticity,and/or ductility at relatively high temperatures. In one embodiment,such hardened cement compositions may be characterized as having anincreased ratio of flexural strength to compressive strength as comparedto conventional cement compositions. As used herein, a “hardened cementcomposition” means a cured or set cement slurry composition.

[0015] The disclosed cement formulations may be advantageously used tocement wellbores in relatively high temperature environments where highstress resistance is required. These include oil/gas, water andgeothermal wells in which high stress conditions exist or in whichcement will be subjected to conditions of high stress including, but notlimited to, those types of wellbores discussed above. Specific examplesof such wells include, but are not limited to, wells having slimholecompletions, highly deviated or horizontal wells, wells exposed tothermal and/or pressure cycling, high perforation density completions,wells completed in formations subject to relatively high overburdenand/or fluid pressures, and wells having junction points between aprimary wellbore and one or more lateral wellbores. Such cement systemsare typically characterized by the ability to provide the ductilityneeded to withstand impacts and shocks of well operations and/orstresses induced by temperature and/or fluid production/injection, whileat the same time providing relatively high compressive strength.

[0016] As disclosed herein, a natural fiber-containing cementing systemmay comprise a hydraulic cement, water, and at least one natural mineralfiber material, such as at least one fibrous calcium silicate material.Examples of suitable calcium silicate fibers include, but are notlimited to, wollastonite pyrophillite, algamatolite, etc. or a mixturethereof. Other cementing additives including, but not limited to,fibers, aluminum silicate (such as a metakaolin), fluid loss additives,set retarders, dispersants, etc. may also be optionally employed.

[0017] In one embodiment using the disclosed cement compositionscontaining natural mineral fiber material, a surprising increase in theratio of flexural strength/compressive strength (i.e., above about 0.35)may be advantageously achieved at downhole temperatures above about 180°F., and particularly at downhole temperatures above about 240° F., witha fibrous mineral content (e.g., wollastonite) of from about 10% toabout 150% by weight of base cement (“BWOC”).

[0018] In one respect, disclosed is a method of cementing within awellbore, including introducing a cement slurry including a hydrauliccement base and a natural mineral fiber into the wellbore; and allowingthe cement slurry to cure within the wellbore to form a hardened cementcomposition within the wellbore; wherein a temperature of at least afirst portion of the well bore is greater than about 180° F.; whereinthe natural mineral fiber is present in the cement slurry in an amountgreater than about 10% by weight of cement, and is also present in thecement slurry in an amount selected to be effective to result in atleast a portion of the cured cement composition having a ratio offlexural strength to compressive strength that is greater than or equalto about 0.35 at the temperature of the at least a first portion of thewell bore that is greater than about 180° F. Examples of natural mineralfibers that may be employed may include, but are not limited to, atleast one of wollastonite, pyrophillite, algamatolite, or a mixturethereof.

[0019] In one embodiment of this method, the natural mineral fiber maybe present in the cement slurry in an amount selected to be effective toresult in at least a portion of the cured cement composition having aratio of flexural strength to compressive strength that is greater thanor equal to about 50% higher than the ratio of flexural strength tocompressive strength of a cured conventional cement composition havingsubstantially the same composition, but without the natural mineralfiber component, at the temperature of the at least a first portion ofthe wellbore that is greater than about 180° F.

[0020] In another embodiment of this method, a temperature of the atleast a first portion of the well bore is less than about 180° F. whenthe cement slurry is introduced into the wellbore and allowed to cure;and further including allowing the temperature of the at least a firstportion of the wellbore to rise above about 180° F.; wherein the naturalmineral fiber is present in the cement slurry in an amount selected tobe effective to result in an increase in the compressive strength of atleast a portion of the cured cement composition when the temperature ofthe at least a first portion of the wellbore is allowed to rise aboveabout 180° F.

[0021] In another embodiment, disclosed is a fiber-containing cementcomposition, comprising a hydraulic cement base and a natural mineralfiber; wherein said natural mineral fiber is present in an amountgreater than about 10% by weight of cement; wherein said natural mineralfiber is also present in said fiber-containing cement composition in anamount selected to be effective so as to result in cement slurry and acured cement composition formed from said cement slurry having a ratioof flexural strength to compressive strength that is greater than orequal to about 0.35 when said cement slurry is exposed to a temperatureof greater than about 180° F.; and wherein said natural mineral fibercomprises at least one calcium silicate natural mineral fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 illustrates ratio of flexural strength to compressivestrength as a function of temperature and wollastonite concentration incement.

[0023]FIG. 2 illustrates values of flexural strength and compressivestrength as a function of temperature and wollastonite concentration incement.

[0024]FIG. 3 illustrates values of flexural strength and compressivestrength as a function of wollastonite concentration in cement at atemperature of 160° F.

[0025]FIG. 4 illustrates values of compressive strength for awollastonite-containing cement as a function of time and temperature.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0026] The disclosed natural fiber-containing cement systems may containnatural mineral fibers such as wollastonite, pyrophillite, algamatolite,mixtures thereof, etc. These cementing systems are useful for, amongother things, cementing operations performed in wellbores havingconditions prone to stress-induced cement failure. As used herein“wellbore stress” includes compressive, tensile and/or shear stresses(such as from shifting formations) that may be experienced by a hardenedcement slurry in a well or wellbore. Such wellbore stress conditionsinclude those described elsewhere herein. In particular, the disclosedcement systems are useful in cementing operations performed in wellboressubject to mechanical, hydraulic and/or thermally induced stresses.Although benefits of the disclosed cement compositions and systems maybe realized in any type of well cementing situation, these compositionsare particularly suitable for use in wells in which conditions of highstress are present or in which cement will be subjected to conditions ofhigh stress including, but not limited to, lateral completions,multi-lateral completions, horizontal wellbores, wellbores forconducting relatively high temperature and/or high pressure fluids,wellbores subjected to high overburden stress or formation shifting,deviated or horizontal wells, wells having one or more doglegs orsidetrack segments, slimhole completions, wells exposed to thermaland/or pressure cycling, wellbores having relatively high perforationdensities, etc.

[0027] The disclosed natural fiber-containing cement compositions may beemployed in wells exposed to high temperatures. These include, but arenot limited to, wells completed in deep and/or high temperatureformations with high temperature fluids, wells used in geothermalapplications, wells used in injection schemes where high temperaturefluids are injected into a formation (such as steam flood wells, cyclicsteam injection wells, etc.), and wells used for producing relativelyhigh temperature fluids (such as fire floods). High temperatures andthermal cycling tend to induce stress on set cement by causing, amongother things, linear and radial expansion and/or contraction of wellboretubulars.

[0028] Surprisingly, the addition of natural mineral fibers (e.g.,wollastonite, pyrophillite, algamatolite, etc.), may be used to producecured ductile/flexible cements having ratios of flexural strength tocompressive strength of greater than about 0.30, alternatively greaterthan about 0.31, alternatively greater than about 0.32, alternativelygreater than about 0.33, alternatively greater than about 0.33,alternatively greater than about 0.34, alternatively greater than about0.35, alternatively greater than about 0.40, alternatively greater thanabout 0.50, alternatively greater than about 0.60, alternatively greaterthan about 0.70, and further alternatively greater than about 0.80 atrelatively high downhole temperatures. In this regard, the disclosedcement compositions may be particularly advantageously employed inwellbores having bottom hole temperatures greater than about 180° F.,alternatively in wellbores having bottom hole temperatures of greaterthan about 240° F., alternatively in wellbores having bottom holetemperatures of greater than about 250° F., alternatively in wellboreshaving bottom hole temperatures of greater than about 300° F.,alternatively in wellbores having bottom hole temperatures of greaterthan about 380° F., and further alternatively in wellbores having bottomhole temperatures of greater than about 400° F.

[0029] It will be understood, however that the compositions may bebeneficially employed in wells having bottom hole temperatures less thanor equal to about 180° F. as well. Further, it will be understood thatthe benefits of the disclosed compositions may be obtained whether theabove-given bottom hole temperatures are the static bottom holetemperature, or a temporary or cyclic elevated temperature induced byproduction or injection operations within a wellbore. Example 1 givesexemplary data regarding calculation of flexural strength to compressivestrength ratios. It should be noted that conventional cementscompositions without the disclosed mineral fiber materials typicallyhave flexural strength to compressive strength ratios of less than about0.30, and typically average around about 0.25.

[0030] In another embodiment, the disclosed compositions may be employedin wellbores in which thermal cycling occurs. By “thermal cycling” it ismeant that a given point of a wellbore is subjected to relatively largemagnitude changes or swings in temperature, such as may be encounteredduring intermittent injection or production of relatively hightemperature or low temperature fluids. In this regard, the disclosedcement compositions may be particularly advantageously employed in wellsin which at least a portion of the wellbore is subjected to recurrent orcyclic temperature changes of greater than about 50° F., alternativelyto recurrent or cyclic temperature changes of greater than about 100°F., alternatively to recurrent or cyclic temperature changes of greaterthan about 150° F., and further alternatively to recurrent or cyclictemperature changes of greater than about 200° F., although thecompositions may be beneficially employed in wells having cyclictemperature changes of equal to or less than about 100° F. as well.

[0031] In another embodiment, benefits of the disclosed naturalfiber-containing cement compositions may be realized in any well inwhich a high pressure differential exists between the interior of thepipe string and the wellbore face, or in which pressure cycling orpressure swings occur. Examples of such situations include those inwhich relatively high pressure well stimulation treatments (such ashydraulic fracturing treatments) are performed, or in which hightemperature production operations cause relatively high annularpressures to develop. In other cases, such conditions may exist wherewellbores penetrate overpressured formations, and/or in which a wellboremay be partially or completely evacuated during completion or productionor later operations. In any event, the disclosed stress-resistant cementcompositions may be particularly advantageously employed when used inwellbores in which a pressure differential of greater than about 2000psi exists between the interior of the pipe string and the wellboreface, more advantageously employed when used in wellbores in which apressure differential of greater than about 3500 psi exists between theinterior of the pipe string and the wellbore face, and mostadvantageously employed when used in wellbores in which a pressuredifferential of greater than about 5000 psi exists between the interiorof the pipe string and the wellbore face, although benefits may also berealized at pressure differentials equal to or less than about 2000 psias well. The effects of such high pressure differentials may be furtherexacerbated by cycling of the pressure, such as may be encountered whenperiodic high pressure well treatments are performed. In this regard,the disclosed cement compositions may be particularly advantageouslyemployed in wellbores subjected to recurrent or cyclic pressure changesof greater than about 1000 psi, alternatively to recurrent or cyclicpressure changes of greater than about 2000 psi, and furtheralternatively to recurrent or cyclic pressure changes of greater thanabout 3000 psi, although the compositions may be beneficially employedin wells having cyclic pressure changes equal to or less than about 1000psi in magnitude as well.

[0032] The preceding embodiments represent only a few of the manywellbore situations in which well cements may be subjected to relativelyhigh mechanical, thermal or hydraulic induced stresses. In this regard,they are exemplary only. It will therefore be understood that benefitsof the disclosed natural fiber-containing cement compositions may berealized in any wellbore cementing application in which a cured or setcement is subjected to relatively high mechanical, thermal or hydraulicstresses. Such wellbore situations include, but are not limited to,annular cement sheaths existing between pipe strings (such as a linercemented within another string of casing or tie-back strings),expandable packers inflated with cement, and/or cement employed torepair casing damage or isolate perforations (such as squeezecementing). Other examples of wellbore cementing in which high stressesmay be encountered include, but are not limited to, cement plugbacks(especially where high pressure differentials and/or mechanical shocksare encountered). Other examples include lateral and/or multi-lateralwellbores having one or more secondary lateral wellbores extending froma primary wellbore. Further information on sources and causes of stressthat may be induced in wellbore cements may be found in Thiercelin etal., “Cement Design Based on Cement Mechanical Response”, SPE Paper38598, pp. 1-23, Oct. 5-8, 1997, which is incorporated herein byreference. Further information about lateral and multi-lateralcompletions may be found in Hogg, “Comparison of Multilateral CompletionScenarios and Their Application,” SPE 38493, pp. 17-27, Sep. 9-12, 1997which is incorporated by reference herein.

[0033] In the practice of the disclosed method and compositions, naturalfiber minerals may be combined with a suitable hydraulic cement ormixture of hydraulic cements and an aqueous base fluid to form acementing slurry. In this regard, any hydraulic cement or mixture ofhydraulic cements suitable for wellbore cementing and compatible with achosen fibrous mineral material may be employed. Examples of suitablehydraulic cement types include, but are not limited to, known hardenablecementitious materials comprising, for example, aluminum, silicon,calcium, oxygen, sulfur or mixtures thereof More specifically, suitablehydraulic cements include gypsum cements, silica cements, high aluminumcontent cements, blast furnace slag-based cements, pozzolona typecements, Portland cements, high alkalinity cements, etc. These hydrauliccements may be used alone or in mixtures. Portland cements are oftenemployed. Typical Portland cements include, but are not limited to, ASTMType I, II, III, V and/or V Portland cements, and API Class A, B, C, Gand/or H Portland cements. However, it will be understood with benefitof this disclosure that other cements and cements containing otheradditives may also be suitably employed, including those describedelsewhere herein. In this regard, a suitable hydraulic cement type ormixture of hydraulic cement types may be selected based on anticipateddownhole conditions, such as temperature, with benefit of thisdisclosure using methods known in the art.

[0034] In the practice of the disclosed method, natural mineral fibersmay be mixed or otherwise combined with a hydraulic cement, water,and/or other desired additives in any order suitable for forming anatural fiber-containing cement slurry. A suitable hydraulic cement maybe mixed with various admixtures including, but not limited to,pozzolan, blast furnace slag, hollow microspheres, nitrogen, andmixtures thereof.

[0035] Examples of natural mineral fibers include, but are not limitedto, carbonate or silicate minerals having a fibrous, aggregate crystalstructure. The term “silicate” as used herein refers to those compoundscontaining silicon, oxygen, and one or more metals. Specific examples ofsuitable fibrous minerals include wollastonite, brucite, trona,sillimanite, sepiolite and pyrophyllite. Specific examples of suitablefibrous silicate minerals include wollastonite, sillimanite, sepioliteand pyrophyllite. Further information on fibrous minerals may be foundin U.S. Pat. No. 5,421,409, and in U.S. Pat. No. 6,230,804, each ofwhich is incorporated by reference herein in its entirety.

[0036] A fibrous mineral-containing cement slurry may contain naturalfibrous mineral in a suitable hydraulic cement such as describedelsewhere herein. In this regard, any fibrous mineral suitable formixture with a hydraulic cement and suitable for increasing ratio offlexural strength to compressive strength of a cured cement compositionto a value of about 0.35 or above at downhole temperatures greater thanabout 180° F. may be employed. Examples include those mineral fibershaving calcium and silicate components, for example, calciummetasilicate natural mineral fibers. In one embodiment, wollastonite maybe employed. Wollastonite is a natural occurring calcium metasilicatemineral which may be found, for example, in metamorphic rock formationsin N.Y. and Calif. Wollastonite is an acicular fiber which tends to forma fibrous, crystalline structure in its aggregate or polycrystallineform. Wollastonite is typically available as very fine or micro-fibershaving diameters similar to that of particles of cement (typically fromabout 25 to about 40 μm) and a fiber length of typically from about 0.4to about 0.6 mm. Wollastonite fibers typically are available in theshape of a acicular particles.

[0037] In the formulation and use of the various cement composition andembodiments disclosed herein, any specific type of wollastonite suitablefor obtaining the desired properties of each embodiment under individualwell conditions may be employed. Suitable wollastonites include, but arenot limited to, wollastonite commercially available as “VANSIL W-10”,available from R. T. Vanderbilt Company of Norwalk, Conn. “VANSIL W-10”may be characterized as acicular shaped micro fibers having fiberlengths of about 0.4-0.6 mm and fiber diameters of about 25-40μm, iscomposed of calcium silicate, having the characteristic of 97.3% passingthrough a 200 mesh screen, a pH of about 10, and a specific gravity ofabout 2.9. Other suitable wollastonites include, but are not limited to,wollastonite available as NYAD G grade from Prescott & Co. ofMississauga, Ontario, Canada. In one embodiment a wollastonite havingbetween about 40% and about 55% CaO and between about 60% and about 45%SiO₂, and alternatively having about 44% CaO and about 50% SiO₂ may beemployed, although it will be understood with benefit of this disclosurethat wollastonites having less than about 40% or greater than about 55%CaO, and having greater than about 60% or less than about 45% SiO₂ maybe employed as well.

[0038] Another suitable type of wollastonite material is a fine mineralwollastonite-based fiber material available from Mineracao Sao JudasLTDA, Sao Paulo, Brazil; and that is available from the Latin AmericanRegion of BJ Services as “MPA-3.” This wollastonite has a specificgravity of about 2.95, is about 75% active, and also contains MgO. Inone embodiment, it is functional over a temperature range of at leastfrom about 0° C. to about 204° C. It may be employed in foamedlightweight, normal and/or heavyweight cement designs. Among the manyadvantageous properties offered by the disclosed cement compositionscontaining fine wollastonite mineral fibers are reduced permeability,enhanced flexural strength development, increased sulfate resistance,minimization of gas migration through the cement matrix, excellentcompressive strength development at temperatures above about 230° F.,and/or production of cement with up to three times higher flexuralstrength/compressive ratios over comparable cement compositions absentthe disclosed mineral fiber additive.

[0039] In the practice of the various embodiments of the disclosedmethod, any amount of natural mineral fiber material suitable forachieving the surprising and advantageous increased ratio of cementflexural strength to compressive strength of the disclosed cementcompositions described herein may be employed. As shown in Example 2 andFIG. 1, surprisingly increased ratios of flexural strength tocompressive strength may be obtained using natural mineral fiber (e.g.,wollastonite) concentrations of greater than about 10% BWOC,alternatively greater than about 12% BWOC, alternatively greater thanabout 15% BWOC at elevated temperatures (e.g., in FIG. 2 at temperaturesgreater than about 250° F. and greater than about 380° F). As shown inFIG. 4, additions of amounts of natural mineral fiber at concentrationsdescribed herein exhibit increasingly advantageous flexural strength tocompressive strength ratios with increasing temperature.

[0040] In various specific embodiments, amount of natural mineral fiberpresent in the disclosed cement compositions for use at the relativelyhigh temperatures disclosed herein may be greater than about 10% BWOC,alternatively greater than about 15% BWOC, alternatively greater thanabout 20% BWOC, alternatively greater than about 25% BWOC, alternativelygreater than about 30% BWOC, alternatively greater than about 35% BWOC,alternatively greater than about 40% BWOC, alternatively greater thanabout 45% BWOC, and further alternatively greater than about 50% BWOC.In yet other embodiments, amount of natural mineral fiber present in thedisclosed cement compositions for use at the relatively hightemperatures disclosed herein may be from about 10% BWOC to about 150%BWOC, alternatively from about 15% BWOC to about 150% BWOC,alternatively from about 20% BWOC to about 150% BWOC, alternatively fromabout 25% BWOC to about 150% BWOC, alternatively from about 30% BWOC toabout 150% BWOC, alternatively from about 35% BWOC to about 150% BWOC,alternatively from about 40% BWOC to about 150% BWOC, alternatively fromabout 45% BWOC to about 150% BWOC, and further alternatively from about50% BWOC to about 150% BWOC. In yet other embodiments, amount of naturalmineral fiber present in the disclosed cement compositions for use atthe relatively high temperatures disclosed herein may be from about x %BWOC to about y % BWOC, where for each respective embodiment the valueof x may be selected from the range of values of from 10 to 149, and acorresponding value of y may be selected from the range of values offrom 11 to 150, with the proviso that y is always greater than x for agiven embodiment. It will be understood with benefit of this disclosure,however, that these compositional ranges are exemplary only, and thatother amounts and ranges of amounts of natural mineral fiber may bebeneficially employed.

[0041] Aluminum silicate is an additive that may be optionally employedin the disclosed compositions. In this regard, any aluminum silicatecomposition suitable for mixture with a hydraulic cement may beemployed. In one example, aluminum silicate may be comprised ofSiO₂/Al₂O₃/Fe₂O₃. An aluminum silicate additive may be kaolin orkaolinite, calcined kaolin or kaolinite (metakaolin), or mixturesthereof. Such aluminum silicate may also be referred to as China Clay.Other suitable forms of aluminum silicate include, but are not limitedto, halloysite, dickite, and nacrite, and mixtures thereof, as well asmixtures of these with materials with kaolin and/or metakaolin. Hydrousform of kaolin is available from Thiele Kaolin Company.

[0042] Further information on suitable aluminum silicates may be foundin “Textbook of Lithology” by Jackson, K. C., 1970, McGraw-Hill, Libraryof Congress No. 72-95810 which is incorporated herein by reference. Asexplained in this reference, in one example kaolins structurally mayconsist of a sheet of silicon-oxygen tetrahedra coordinated with a sheetof aluminum-oxygen-hydroxide octahedra. The resultant double sheet istypically electrostatically neutral so that no additional ions arerequired. The various minerals of the group may differ in the stackingpatterns of these double sheets.

[0043] Aluminum silicates may have the content of silica may be betweenabout 75% and about 25%, alternatively between about 65% and about 52%by weight, and the content of alumina may be between about 25% and about75%, alternatively between about 35% and about 48% by weightrespectively, although other silica and alumina contents are possible,including silica contents greater than about 75% and less than about 25%by weight, and alumina contents less than about 25% and greater thanabout 75% by weight. Aluminum silicates may contain trace amounts offerric oxide. In this regard, any ferric oxide fraction present may bepresent in an amount less than about 1% by weight of aluminum silicate,although fractions greater than about 1% are also possible.

[0044] Aluminum silicate may have a particle size of between about 0.5μM and about 2 μM and a specific gravity of greater than or equal to2.2, and alternatively of about 2.5, although sizes and specificgravities outside these ranges are also possible. In this regard,smaller or more fine particles of aluminum silicate may be useful insituations requiring greater reactivity. Aluminum silicate may beemployed in the form of kaolin or calcined anhydrous kaolin(metakaolin), such as metakaolin or high reactivity metakaolin (“HRM”).Examples of HRM aluminum silicates include, but are not limited to,those commercially available as “METAMAX” and, in finer form, as“METAMAX EF”, both available from Engelhard Corporation, SpecialtyMinerals and Colors of Iselin, N.J. “METAMAX” may be characterized ascalcined anhydrous Kaolin Al₂O₃•SiO₂, and has an average particle sizeof about 1.5 μM, is composed of 97% SiO₂+Al₂O₃+Fe₂O₃, with a specificgravity of about 2.5, a maximum wet screen residue of about 0.35% at+325 mesh, a pH of about 4.5-6.5, a maximum free moisture content ofabout 1.0, a loose bulk density of about 18 lbs/ft³, a tamped bulkdensity of about 32 lbs/ft³, and a specific gravity of about 2.5. Incomparison, “METAMAX EF” has an average particle size of about 0.5 μM,is composed of 98% SiO₂+Al₂O₃+Fe₂O₃, and has a specific gravity of about2.5, with a similar pH and free moisture content as “METAMAX.” It willbe understood with benefit of this disclosure that “METAMAX” and“METAMAX EF” are merely given as specific examples of suitable aluminumsilicates, and that other aluminum silicates may be employed as well.

[0045] In the practice of the disclosed method, natural mineral fibersmay be mixed with hydraulic cement to form a fiber-containing cementsystem or composition. To form a cement slurry, fiber-containing cementsystem or composition may be mixed with fresh water, but may also bemixed with sea water or any other suitable aqueous-based fluid includingbut not limited to formation brine, KCl water, NaCl water, sea water,drill water, drilling mud or mixtures thereof. However, it will beunderstood with benefit of the present disclosure that one or morenatural mineral fibers may be added at any point in a cement slurrymixing process, including after a hydraulic cement has been mixed withan aqueous based fluid, and/or optionally mixed with an aqueous basefluid prior to mixing with a hydraulic cement.

[0046] The water requirement of a cement slurry may be varied to achievedesired density and pumpability. In this regard any amount of watersuitable for forming a natural mineral fiber-containing cement slurrysuitable for placement in a wellbore may be employed. For example, inone embodiment, a natural fiber-containing cement slurry density may beformulated to be between about 11 lbm/gal and about 19 lbm/gal,alternatively between about 16.0 lbm/gal and about 15.0 lbm/gal, andfurther alternatively from about 15.5 lbm/gal to about 16.5 lbm/gal.However, any other slurry density suitable for use in a wellbore may beemployed, including less than about 11 lbm/gal or greater than about 19lbm/gal. The system may also be formulated with lightweight additivesincluding, but not limited to, additives such as microspheres and/orfoamed with nitrogen gas or other suitable energizing phase to achievelower densities, for example, to obtain densities as low as about 0.96g/cm³ (8 lbm/gal).

[0047] In embodiments of the disclosed methods and compositions, otheradditives, including any suitable cementing additives known to those ofskill in the art may be employed in the formulation of a naturalfiber-containing cement slurry. Optional additives may be used, forexample, to further vary characteristics of a natural fiber-containingcement slurry, including to further vary viscosity, further controlfluid loss, further immobilize water between particles, to furtherimpart variable thixotropic properties to a cement slurry, to varytransition time, etc. Examples of possible additives include, but arenot limited to, accelerators, dispersants, viscosifiers, fluid losscontrol agents, set retarders, low density additives, weighting agents,thinners, foamers, lost circulation materials, energizing gases (such asnitrogen gas, air, etc.). Thus, a cement slurry may be formulated, forexample, to meet a given situation and to provide a reduced transitiontime while at the same time providing a density compatible withformation pressure gradients in order to avoid cement loss to theformation. For example, embodiments of the disclosed cement slurries mayinclude lesser amounts of accelerator additives for use at relativelyhigher downhole temperatures.

[0048] In one embodiment, one or more additives suitable for decreasingtransition time may optionally be employed. Examples of such additivesinclude gypsum, calcium chloride, sodium silicate, metasilicate,metakaolin, or mixtures thereof. As a particular example, a naturalfiber-containing cement may include between about 1% and about 15%, andalternatively between about 1% and about 10% gypsum BWOC, such as “A-10”gypsum available from BJ Services. However, amounts greater than about15% gypsum BWOC and less than about 1% gypsum BWOC are also possible.

[0049] A cement slurry embodiment may also include optional cement fluidloss control additives, especially when low pressure or “thief” zonesare suspected to be present. Such additives include any additive/ssuitable for controlling fluid loss from a cement slurry prior tosetting. Typical fluid loss control additives include, but are notlimited to, materials such as hydroxyethyl cellulose (“HEC”), HECblends, carboxymethyl hydroxyethyl cellulose (“CMHEC”), CMHEC blends,polyethylene imine (“PEI”), copolymers of acrylamide and acrylic acid,polyvinyl alcohol (“PVA”), PVA blends, etc. Other examples of suitableadditives include, but are not limited to, 2-acrylomido, 2-methylpropane sulfonic acid, (“AMPS”) copolymers, terpolymers or mixturesthereof. Other fluid loss control additives may also be employed. Suchfluid loss control additives may be employed in an amount of from about0.1% to about 3%, alternatively from about 0.1% to about 2%, and in oneembodiment in an amount of from about 0.1% to about 1.5% BWOC, althoughother amounts (such as amounts greater than about 3% BWOC) are alsopossible. In one embodiment, between about 0.1% and about 3.0%, andalternatively from about 0.1% to about 1.5% of “FL-33” fluid losscontrol additive BWOC (available from BJ Services) may be employed.

[0050] Any additive/s suitable for controlling fluid flow may also beoptionally employed including, but not limited to, polyvinylalcohol-based anti-fluid flow additives. For example, in one embodimenta polyvinyl alcohol fluid flow additive (such as “BA-10” available fromBJ Services) may be used in an amount of between about 0.1% and about3.0%, alternatively from about 0.1% to about 1.5% BWOC, although otheramounts are possible.

[0051] Accelerators may also be optionally employed. In this regard, anyadditive/s suitable for well cementing may be used including, but notlimited to, calcium chloride potassium chloride, sodium chloride,seawater, sodium silicate, sodium metasilicate, metakaolin or mixturesthereof. In one embodiment, between about 0.1% and about 4%,alternatively from about 0.1% to about 2% of “A-7” calcium chloride BWOC(available from BJ Services) may be employed in formulating a slurry,although other amounts are possible.

[0052] The disclosed natural fiber-containing cement compositions mayalso be optionally formulated to contain consolidating fibers, such asnylon or polypropylene fibers, to reduce the potential for cement debrisformed under high stress conditions. Examples of suitable consolidatingfibers include, but are not limited to, at least one of carbon fibers,nylon fibers, polypropylene fibers, or a mixture thereof.

[0053] Any dispersant additive/s suitable for facilitating the mixing ofwet and dry materials in a slurry and/or activating dry materials mayalso be used including, but not limited to, dispersants such asnaphthalene sulfonate, ethoxylated napthalene sulfonate orketone-acetone sulfonate. Such additives may be particularly useful, forexample, when lower water to cement ratios are employed. In oneembodiment, between about 0.1% and about 3%, alternatively from about0.1% to about 1.0% of acetone sulfonate, ethoxylated napthalenesulfonate, or naphthalene sulfonate (such as “CD-33,” “CD-32” or“CD-31”, respectively, available from BJ Services) BWOC is used,although other amounts are possible.

[0054] Low density additives may also be optionally employed. In thisregard, any additives suitable for lowering slurry density may be usedincluding, but not limited to, sodium silicate, sodium metasilicate,hollow microspheres, bentonite or mixtures thereof. In one embodiment,between about 1% and about 75%, alternatively from about 1% to about 50%of a lightweight additive such as hollow ceramic microspheres availableas “LW-6” BWOC (available from BJ Services) may be employed informulating a slurry, although other amounts are possible.

[0055] Set retarders may also be optionally used. Any set retardercomposition suitable for retarding or otherwise delaying the setting ofa natural fiber-containing cement, such as for increasing pumping timeof a cement slurry, may be used. Examples include, but are not limitedto, lignosulfonates, sugars, phosphonates, or mixtures thereof. In oneembodiment, between about 0.1% and about 3%, alternatively from about0.1% to about 1.0% of a sodium lignosulfonate cement retarder “R-3” BWOC(available from BJ Services) may be employed as a set retarder, althoughother amounts are possible.

[0056] It will be understood with the benefit of this disclosure that acement slurry may also contain other conventional additives includingbut not limited to additives for controlling free water or solidseparation, silica fume, glass or ceramic microspheres, perlite,biopolymers, etc.

[0057] When so desired, a cement slurry containing natural mineral fibermaterials may be foamed utilizing a foaming agent, optional stabilizer,and an energizing phase. In this regard, any foaming agent and/orstabilizer suitable for creating a stable foamed naturalfiber-containing cement slurry, may be employed in any amount suitablefor obtaining a foamed cement slurry. In the case of salt water basedcement slurries, a foaming agent may include, but is not limited to,oxyalkylated sulfates or ethoxylated alcohol sulfates, or mixturesthereof. In one embodiment “FAW-20” ethoxylated alcohol sulfate foamingagent available from BJ Services is utilized. Suitable salt waterstabilizers include, but are not limited to, polyvinyl alcohol, sodiumsilicate, or mixtures thereof. In one embodiment, a polyvinyl alcoholstabilizer known as “BA-10” and available from BJ Services is used. Inthe case of fresh water based cement slurries, a foaming agent mayinclude, but is not limited to, oxyalkylated sulfates or ethoxylatedalcohol sulfates, or mixtures thereof In one embodiment, “FAW-20”foaming agent available from BJ Services may be utilized. Suitable freshwater stabilizers include, but are not limited to, polyvinyl alcohol orsodium silicate, or mixtures thereof. “BA-10” stabilizer available fromBJ Services may be used.

[0058] Any energizing phase composition suitable for forming a foamedfibrous mineral-containing cement may be employed including but notlimited to gaseous material such as carbon dioxide, nitrogen, liquidpetroleum gases (such as liquefied natural gas and liquefied petroleumgas, etc.), air or a mixture thereof. An energizing phase may be addedto a mixture of cement, aqueous fluid, surfactant and stabilizer. Theslurry density may be controlled with benefit of this disclosure byadjusting the amount of energizing. phase added to an unfoamed cementslurry. For example, in one embodiment the density of a cement slurrymay be adjusted from about 8 to about 15 lbs/gal by adding from about1500 to about 25 standard cubic feet (SCF) of nitrogen gas at standardconditions per barrel (bbl) of unfoamed cement slurry, although anyother amounts suitable for obtaining a foamed cement slurry arepossible.

[0059] One or more defoaming additives may also be optionally used withnatural mineral fiber-containing foamed cement slurries to preventfoaming during mixing and pumping of a foamed slurry. In this regard,any defoaming additive suitable for cementing operations may be employedincluding, but not limited to, glycol, alcohols or silicones, ormixtures thereof. In one embodiment, “FP-12L” defoaming additiveavailable from BJ Services may be employed in an amount of from about0.01 to about 0.5 gallons per sack (“GPS”) concentration, in anotherembodiment from about 0.05 to about 0.1 GPS concentration, althoughother amounts are possible.

[0060] In either salt water or fresh water based cement slurries, anysuitable energizing phase, including but not limited to nitrogen, CO₂,air, natural gas or mixtures thereof may be employed in a sufficientamount to achieve the desired density of cement, for example in anamount of between about 10 SCF/bbl and about 2000 SCF/bbl at standardconditions, in one embodiment between about 100 SCF/bbl to about 1000SCF/bbl, although other amounts are possible. In one embodiment nitrogenmay be employed.

[0061] It will also be understood with benefit of this disclosure thatthe disclosed natural fiber-containing cement operations may be employedwith benefit in cementing operations performed in wells havingconventional levels or risk of stress induced cement failure, or inwells in which situations other than those described herein createstress in set cement. In this regard, it will be understood that“cementing operations” as used herein means any type of wellborecementing application known in the art including, but not limited to,long string cementing, liner cementing, inflatable/external packercementing, squeeze cementing, plug back cementing, temporary plugcementing, casing repair cementing, zone isolation cementing, etc. Suchoperations include, but are not limited to, drilling, completion andremedial cementing operations, including those performed on existingcompleted wellbores, as well as those cementing operations performedduring well abandonment operations.

[0062] Furthermore, it will be understood with benefit of thisdisclosure that although exemplary ranges and amounts of hydrauliccement, fibrous minerals and additives are described and illustratedherein, any other amounts of these components and/or other additives maybe suitably employed where the benefits of the disclosed naturalfiber-containing cement systems may be realized as described elsewhereherein. It will also be understood that although specific embodiments ofcementing procedures using natural fiber-containing cement slurries havebeen described herein, a natural fiber-containing cement slurry may bemixed, pumped, spotted, or otherwise introduced into a wellbore and/orwellbore annulus in any manner known to those of skill in the art.Furthermore, a natural fiber-containing cement slurry may be formulatedwith benefit of this disclosure in any suitable manner known to those ofskill in the art including, but not limited to, by continuous mixing,batch mixing, etc.

EXAMPLES

[0063] The following examples are illustrative and should not beconstrued as limiting the scope of the invention or claims thereof.

Example 1

[0064] Table 1 shows properties for several cement compositions, withand without natural mineral fibers added. With the addition of certainother minerals, (such as kaolinite, either natural or calcined),relatively high ratios of flexural strength to compressive strength mayalso be produced. This may be seen in Table 1 for cement compositionscontaining both natural mineral fibers (e.g., wollastonite oralgamatolite) and meta kaolinite. TABLE 1 Flexural Meta- CompressiveStrength FS:CS Density, Kaolinite Wollastonite Algamatolite Strength (23h @ Ductility Slurry PPG Fiber Fiber Fiber (23 h @ 135° F.) 135° F.)Ratio “A” +0.4% Na M- 13.3 — — — 1100 PSI 295 PSI 0.27 Silicate “A”+0.4% Na M- 13.3 15% 35% —  600 PSI 196 PSI 0.33 Silicate “A” +0.4% NaM- 13.3 15% — 35%  900 PSI 314 PSI 0.35 Silicate

Example 2

[0065]FIG. 1 shows ratio of flexural strength/compressive strength as afunction of wollastonite concentration in % BWOC at 250° F. and 380° F.Table 1 shows the surprising increase in the ratio of flexuralstrength/compressive strength (“FS/CS ratio”), with increasing amountsof wollastonite and increasing temperatures, e.g., above 200° F., bymore than 50% as compared to conventional cements (having an averageFS/CS ratio 0.25). FIG. 2 shows compressive strength (“CS”) and flexuralstrength (“FS”) values as a function of wollastonite concentration in %BWOC at 250° F. and 380° F. The wollastonite employed in this example iswollastonite-based fiber material from Brazil, and is available from theLatin American Region of BJ Services as “MPA-3.

[0066] It may be seen that with benefit of this disclosure, and usingthe methodology of this example, an appropriate amount of naturalmineral fiber (e.g., wollastonite) greater than about 10% BWOC may beselected for a particular downhole temperature to achieve an surprisingand advantageous ratio of flexural strength to compressive strength(e.g., ratio greater than about 0.35).

Example 3

[0067] Diffractograms were obtained for three cements with 40% silicaflour and 0%, 50% and 100% wollastonite respectively cured at 380 ° F.for 48 hours, along with the diffractogram of wollastonite. None of thecement samples appeared to contain wollastonite. Quartz was detected inall samples. Tobermorite (Ca5Si6O16 (OH) 2×4H2O) was the principalcement component detected in all samples. The relative abundance oftobermorite increases with increasing “wollastonite” content. Xonotlite(Ca6Si6O17 (OH) 2) was present in both samples which contained“wollastonite”. Xonotlite is a common hydrothermal cement component.Diopside (a pyroxene mineral) was detected in the two samples, whichcontain “wollastonite”. Calcium aluminum ferrite (possiblybrownmillerite) was detected in a minor quantity in the sample, whichcontained no “wollastonite”.

[0068] While not wishing to be bound by theory, it is believed thatnatural mineral fibers (e.g., wollastonite, pyrophillite, algamatolite,mixtures thereof, etc.) become hydraulically active (reacting withcement) as temperatures increase above about 180° F. Additionally, attemperatures above about 240° F., it is believed that these fibers mayparticipate in reactions that counteract strength retrogression ofcements due to their higher silica content than oil well cements.Natural fibers at these temperatures react with cement faster,increasing the Ca/Silica ratio of cement during hydration and settingprocess, making their use advantageous at higher temperatures where “setcement” typically becomes more brittle. Dilution of cement (reduction ofcement content) by the addition of natural fibers and higher amount ofTobermorite and faster generation of Xonolite (which has strength 25%lower than Tobermorite) are believed to be one cause for this incrementin ductility. Depending on the nature of the fiber used and the testingtemperature, other crystalline phases (Pectolite, Scawtite, Truscotite,etc.) may also be formed, but generally the strength effects is similar.

Example 4

[0069] Flexural strength and compressive strength were also evaluated attemperatures below 160 ° F. for varying concentrations of wollastonite.As shown in FIG. 3, in this temperature range (below about 170° F.), theFlexural Strength varies proportionally to the compressive strength asshown in FIG. 3. In FIG. 3, “FS” denotes flexural strength, and “CS”denotes compressive strength.

Example 5

[0070] In this example, a 15.0 pound per gallon (“PPG”) cement slurrycontaining “A” hydraulic cement base, 35% BWOC silica, and natural 100%BWOC mineral micro fiber (wollastonite) was cured at a relatively lowtemperature (below 180° F.), but was later exposed to highertemperatures. As shown in FIG. 4, the hydration process restarted,allowing the cement to “reactivate” or regain compressive strength,instead of losing compressive strength due to retrogression. Thisbehavior is advantageous for thermal cycling, such as seen in geothermalwells and cyclic steam injection wells, where temperature cycles tend todestroy the integrity of the cement matrix due to casingexpansion/contraction cycles (tangential forces) and strengthretrogression (radial-compressional forces).

[0071] Although particular exemplary embodiments of the disclosedcompositions have been described and illustrated herein, it will beunderstood with benefit of this disclosure that benefits of thedisclosed cement compositions and cementing methods may be realized inany type of wellbore cementing application, including in completion,remedial, workover, and/or abandonment cementing applications usingcementing methods known in the art. Examples of specific applicationsinclude, but are not limited to, cementing casing and liner strings,inflatable packers, squeezing perforations and casing leaks, etc.

[0072] While the invention may be adaptable to various modifications andalternative forms, specific embodiments have been shown by way ofexample and described herein. However, it should be understood that theinvention is not intended to be limited to the particular formsdisclosed. Rather, the invention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theinvention as defined by the appended claims. Moreover, the differentaspects of the disclosed compositions and methods may be utilized invarious combinations and/or independently. Thus the invention is notlimited to only those combinations shown herein, but rather may includeother combinations.

1-19. (canceled)
 20. A fiber-containing cement composition, comprising ahydraulic cement base and a natural mineral fiber; wherein said naturalmineral fiber is present in an amount greater than about 10% by weightof cement; wherein said natural mineral fiber is also present in saidfiber-containing cement composition in an amount selected to beeffective so as to result in a cement slurry and a cured cementcomposition formed from said cement slurry having a ratio of flexuralstrength to compressive strength that is greater than or equal to about0.35 when said cement slurry is exposed to a temperature of greater thanabout 180° F.; and wherein said natural mineral fiber comprises at leastone calcium silicate natural mineral fiber.
 21. The fiber-containingcement composition of claim 20, wherein said calcium silicate naturalmineral fiber comprises at least one of wollastonite, pyrophillite,algamatolite, or a mixture thereof.
 22. The fiber-containing cementcomposition of claim 20, wherein said calcium silicate natural mineralfiber comprises wollastonite.
 23. The fiber-containing cementcomposition of claim 22, wherein said hydraulic cement base comprisesPortland Cement.